In recent years, along with development and spread of electronic equipments represented by an interruptible power, mobile communication instruments and portable instruments, hybrid automobiles which can be driven by both energies of gasoline and electricity, and electric vehicles, the demand for improving properties of a storage battery device such as a secondary battery has been remarkably increasing. Specifically, a high performance battery having a high output, a high capacity and an excellent cyclic property is desired.
Such a storage device usually comprises electrodes, a non-aqueous electrolyte, a separator, etc. as the main members. In general, an electrode for a storage battery is produced by applying an electrode mixture for a storage battery device having an electrode active material, an electrically conductive material and a polymer as a binder dispersed in an organic solvent or water on a surface of a metal foil as a current collector, followed by drying. If the binder is incapable of fixing a sufficient amount of the battery active material on the electrode, a battery having a large initial capacity cannot be obtained, and e.g. by repeating charge and discharge, the battery active material falls away from the electrode, whereby the capacity of the battery deteriorates.
The binder for such an electrode mixture for a storage battery device is mainly classified into an organic solvent binder wherein a polymer is dissolved in an organic solvent and a water binder wherein a polymer is dissolved or dispersed in water. Particularly in recent years, attention has been paid to the water binder, since costs for producing an electrode by use of an organic solvent can be reduced, and environmental burden and working environment can be improved.
The water binder is supplied as a composition having a polymer dispersed in an aqueous medium. Particularly, many water type binders are produced by an emulsion polymerization method using an emulsifying agent, since they are excellent in productivity, molecular weight and dispersion stability.
Patent Document 1 discloses a binder composition for a lithium ion secondary battery electrode, which comprises a copolymer of styrene and 1,3-butadiene or a copolymer of styrene, methyl methacrylate and 2-ethylhexyl acrylate, prepared by emulsion polymerization using a sodium alkyl diphenyl ether disulfonate as an emulsifying agent.
Patent Document 2 proposes a fluorinated polymer as a binder for a storage battery device which is excellent in the oxidation resistance. Further, among fluorinated polymers, proposed is a copolymer of tetrafluoroethylene and propylene or a copolymer of tetrafluoroethylene, propylene and vinylidene fluoride as a binder which is excellent in the flexibility and the adhesion, as compared with polytetrafluoroethylene.